There is an image processing method disclosed in the related art (see patent reference 1) adopting the concept of the uniform noise space achieved through square root gradation conversion so as to convert shot noise, which increases by a factor of the square root of the signal intensity, to noise remaining constant relative to the signal intensity. This method, through which the noise fluctuation width, i.e., the standard deviation, is indicated by a single representative value regardless of the gradation, facilitates noise profiling.
For instance, if image data are filtered in a uniform noise space with a σ filter (non-patent reference 1) through which noise is removed by averaging pixel values whose differences are equal to or less than a constant multiple of the standard deviation σ or with a bilateral filter (nonpatent reference 2) through which noise is removed through Gaussian blur by using the ratios of pixel value differences relative to the standard deviation σ as an argument in conjunction with spatial Gaussian blur, the noise can be accurately removed in a manner optimal for the particular image structure, since the standard deviation value is not dependent upon the luminance. If the noise is to be removed without using such a uniform noise space, a process of generating a noise profile by determining the standard deviation in correspondence to each luminance level needs to be executed as disclosed in patent reference 2.
A color space in which the luminance and chrominance are expressed by taking into consideration the human visual system, is often used in image processing. Under normal circumstances, a uniform color space such as L*a*b* is routinely used, since such a uniform color space in which a color signal is expressed in a manner closer to human visual perception, is considered to assure superior color reproduction.    Patent reference 1: Japanese Laid Open Patent Publication No. H7-92588    Patent reference 2: U.S. Pat. No. 5,923,775    Nonpatent reference 1: J. S. Lee “Digital Image Smoothing and the Sigma Filter” Computer Vision, Graphics and Image Processing, Vol. 24, pp 255˜269, 1983    Nonpatent reference 2: C. Tomasi et al. “Bilateral Filtering for Gray and Color Images,” Proceedings of the 1998 IEEE International Conference of Computer Vision, Bombay, India